MusicBrainz Picard is a very smart opensource tool which creates a "footprint" of your song and looks up all the information. You can save then just the tags, or also rename or even move the files. It also does the cover pic somehow, but I haven't checked it out yet.
It works not with each and every single song, depending on how you got your mp3, but there are ways to give the app a hand.
thanks! I love it.

Btw, it is an example of the semantic web, applied to music. Will write about that, once I got a better idea.